Method for manufacturing sintered component, sintered component, and drill

ABSTRACT

Provided is a method for manufacturing a sintered component, which can suppress occurrence of edge chipping when a through-hole is formed in a powder-compact green body and also has a good productivity. The method for manufacturing a sintered component includes a molding step of press-molding a raw material powder containing a metal powder and thus fabricating a powder-compact green body; a drilling step of forming a hole in the powder-compact green body using a drill; a sintering step of sintering the powder-compact green body after drilling, wherein the drill used for drilling has a circular-arc shaped cutting edge on a point portion thereof.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a sinteredcomponent, a sintered component and a drill for drilling apowder-compact green. In particular, the present invention relates to amethod for manufacturing a sintered component, which can suppressoccurrence of edge chipping when a through-hole is formed in apowder-compact green body and also has a good productivity.

BACKGROUND ART

Sintered bodies (sintered alloys) obtained by sintering green bodiesmade of a metal powder, such as an iron powder, are used for automobileparts or machine parts. For example, such sintered alloy components(hereinafter, simply referred to as a “sintered component”) includesprockets, rotors, gears, rings, flanges, pulleys, vanes, bearings andthe like. In general, the sintered components are manufactured bypress-molding a raw material powder containing a metal powder tofabricate a powder-compact green body (powder compact body) and thensintering the powder-compact green body. After sintering, if necessary,the sintered components are subjected to machining as a finishprocessing.

However, as sintered components, components are known, in which athrough-hole (open hole), which extends therethrough, or a blind hole,which does not extend therethrough, is formed. For example, a componentis known, in which a through-hole (e.g., an oil hole) is formed toextend from an outer circumferential surface thereof to an end surfaceor an inner circumferential surface. In such a component, a through-holecannot be integrally formed in a powder-compact green body duringmolding, and thus after sintering, drilling is performed using a drill(see Patent Document 1).

As drills used for drilling, a drill, in which a cutting edge on a pointportion thereof has a V-shaped projection shape, is typical. In the caseof sintered carbide drills, a point angle of the cutting edge is in theorder of 130° to 140°.

CITATION LIST Patent Document

-   Patent Document 1: Japanese Patent Application Publication No.    2006-336078

SUMMARY OF INVENTION Technical Problem

In the case where drilling using a drill is performed on a sinteredcomponent, there is a problem in that drilling after sintering isdifficult and has a low productivity.

The sintered component is hard, since metal powder particles therein arediffusion-bonded and alloyed with each other by sintering, therebyforming a strong bonding therebetween. Accordingly, when drilling usinga drill is performed on the sintered component, a cutting resistance ishigh and thus the drill is difficult to penetrate into the sinteredcomponent. Therefore, cutting is difficult, a long time is required formachining and also a tool life is decreased. Also, since a resistanceupon biting of the drill is also high, it is difficult to obtain astable precision of the machined hole, due to the fact that a rotationaxis of the drill is likely to be shaken and the like.

In addition, since a cutting resistance is high and also a thrust loadis high, when a through-hole is formed, burrs are likely to occur alongan opening edge on the exit side, through which the drill comes out. Theburrs occur since if a thickness of a bottom of a hole when a drillpenetrates becomes so thin that a strength of the bottom against athrust load cannot be maintained, the bottom is deformed and pushed outto the exit side. The occurred burrs have to be removed in thesubsequent step and thus time and efforts are required for such anoperation. It is difficult or impossible to remove the burrs dependingon locations where the burrs occur.

Therefore, in view of reduction of manufacturing costs, it is desirableto improve productivity in manufacturing a sintered component.

Accordingly, the preset inventors have considered performing drilling ona powder-compact green body before sintering by a drill so that athrough-hole is formed in the powder-compact green body in advance,instead of performing drilling on a sintered component after sinteringby a drill. The powder-compact green body is obtained by onlyagglomerating a raw material powder by molding, and thus metal powderparticles therein are mechanically adhered with each other, not stronglybonded to each other as in the sintered body. Therefore, when drillingis performed on the powder-compact green body before sintering, cuttingcan be easily performed and also a cutting resistance (thrust load) canbe significantly reduced, since bonding between metal powder particlesthereof is weak. When drilling is performed on the powder-compact greenbody, a through-hole is formed as metal powder particles are cut whilebeing scrapped by the drill. However, if drilling is performed on thepowder-compact green body, when a through-hole is formed, so-called edgechipping that an opening edge on the exit side, through which the drillcomes out, is chipped is likely to occur.

Accordingly, one of objects of the present invention is to provide amethod for manufacturing a sintered component, which can suppressoccurrence of edge chipping when a through-hole is formed in apowder-compact green body and also has a good productivity. Anotherobject of the present invention is to provide a sintered component,which has a good productivity. Further another object of the presentinvention is to provide a drill, which can suppress occurrence of edgechipping when a through-hole is formed in a powder-compact green body.

Solution to Problem

A method for manufacturing a sintered component according to one aspectof the present invention includes a molding step, a drilling step and asintering step. The molding step is configured to press-mold a rawmaterial powder containing a metal powder and thus to fabricate apowder-compact green body. The drilling step is configured to form ahole in the powder-compact green body using a drill. The sintering stepis configured to sinter the powder-compact green body after drilling.The drill used for drilling has a circular-arc shaped cutting edge on apoint portion thereof.

A sintered component according to one aspect of the present invention isa sintered component having a hole formed therein. In the sinteredcomponent, an inner circumferential surface of the hole has a satinfinish shape.

A drill according to one aspect of the present invention is a drill fordrilling a work material. The work material is a powder-compact greenbody fabricated by press-molding a raw material powder containing ametal powder. The drill has a circular arc-shaped cutting edge on apoint portion thereof.

Advantageous Effects of Invention

The method for manufacturing a sintered component as described above cansuppress occurrence of edge chipping when a through-hole is formed in apowder-compact green body and also has a good productivity. The sinteredcomponent as described above has a good productivity. The drill asdescribed above can suppress occurrence of edge chipping when athrough-hole is formed in a powder-compact green body.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view comparing between a case where drilling isperformed using a drill having a circular arc-shaped cutting edge and acase where drilling is performed using a drill having a V-shaped cuttingedge.

FIG. 2 is a process explanatory view explaining a method formanufacturing a sintered component according to an embodiment.

FIG. 3 is a schematic view explaining an example of a drill according toan embodiment.

FIG. 4 is a microscope image showing an exit of a through-hole in thecase where the through-hole is formed using an R drill in Test Example1.

FIG. 5 is a microscope image showing an exit of a through-hole in thecase where the through-hole is formed using a V drill in Test Example 1.

FIG. 6 is a graph showing a relationship of a rake angle, a thrust loadand a torque in the case where a through-hole is formed using an R drillin Test Example 2.

FIG. 7 is a microscope image showing an inner circumferential surface ofa through-hole of a powder-compact green body fabricated in Test Example3.

DESCRIPTION OF EMBODIMENTS

[Explanation of Exemplary Embodiments of the Invention]

The present inventors have studied technologies for improvingproductivity of a sintered component and as a result have found that theproductivity can be enhanced if drilling using a drill is performed on apowder-compact green body before sintering, not after sintering. Thereason is that when drilling is performed on the powder-compact greenbody, cutting is easily performed and a cutting resistance (thrust load)is significantly reduced, since bonding between metal powder particlesis weak. Further, as compared with a conventional case where drilling isperformed after sintering, a machining time can be reduced, a precisionof a machined hole can be enhanced and also a too life can besignificantly improved. Further, when drilling using a drill isperformed on the powder-compact green body, burrs hardly occur. Even ifburrs have occurred, the burrs can be easily removed by an air blower orthe like, and thus time and efforts required for a bur removingoperation can be reduced.

Also, the present inventors have further progressed the above studiesand as a result have found that occurrence of edge chipping when athrough-hole is formed can be suppressed by devising a shape of a drillused for drilling of the powder-compact green body, in particular ashape of a cutting edge on a point portion thereof. Specifically, it hasbeen found that occurrence of edge chipping can be suppressed by forminga shape of the cutting edge into a circular-arc shape (R-shape).

A mechanism of occurrence of edge chipping is thought as follows. Thepowder-compact green body is soft since bonding between metal powderparticles is weak. Therefore, if a thickness of a bottom of a holeformed by a drill when the drill penetrates becomes so thin that astrength of the bottom against a thrust load cannot be maintained, thebottom is not cut by the drill but comes off (pushed out) to an exitside before the drill passes through the bottom. When the bottom is notcut but comes off, the vicinity of the bottom also collapses, and as aresult, chipping occurs on an edge of an opening on the exit side,through which the drill comes out.

The reason that occurrence of edge chipping can be suppressed by using adrill having a circular arc-shaped cutting edge (hereinafter, oftenreferred to as an “R drill”) is thought as follows. Views on the leftside of FIG. 1 show a drill (R drill) 10, in which a shape of a cuttingedge thereof is a circular arc shape, and a work material(powder-compact green body) G, on which drilling has been performedusing the R drill 10. In order to make the explanation easier tounderstand, the R drill 10 illustrated in FIG. 1 is simply shown byomitting grooves and the like. As shown in a view on the left upper sideof FIG. 1, the R drill 10 is configured so that a shape of the cuttingedge 110 is a semi-circular shape, a central angle α of a circular arcdefining the cutting edge 110 is 180°, and also a radius R of thecircular ac is equal to a radius d/2 of the drill. The R drill 10 isconfigured so that a length h of a point portion 100 along an axialdirection of the drill is equal to the radius R of the circular arc. Thepoint portion 100 is a portion ranging from a point (apex) of thecutting edge 110 to outer corners 120.

As shown in two views on the left lower side of FIG. 1, when drilling isperformed on a powder-compact green body G using the R drill 10 (anoutlined arrow in the figure indicates a feed direction of the drill),the shape of the cutting edge 110 is transferred to the powder-compactgreen body G, so that a hole, of which a bottom surface has a crosssection of a circular arc (semi-circular) shape, i.e., a hemispherichole is formed in the powder-compact green body G. Since the shape ofthe cutting edge 110 is the circular arc (semi-circular arc) shape, asindicated by solid arrows in the figure, a thrust load is exerted in aradially dispersed state on the R drill 10. Also, in the powder-compactgreen body G, as indicated by solid arrows in the figure, the bottom ofthe hole having the hemispheric shape bears a thrust load from the drillby a spherical surface thereof, and thus has a higher resistance todeformation and a higher strength.

In other words, when drilling is performed on the powder-compact greenbody G using the R drill 10, the thrust load itself is lower and alsothe trust load exerted on the bottom is dispersed. Accordingly, a stressconcentration is reduced and the bottom is hardly chipped.

Also, as shown in the view on the left lower side of FIG. 1, a maximathickness Ht of the bottom of the hole is defined as a length from asurface of the thinnest portion (deepest portion) of the bottom to asurface of the thickest portion of the bottom. The maximum thickness Htof the bottom of the hole is equal to a length h of the point portion100 and is increased as the length h of the point portion 100 isincreased. The R drill 10 can have an increased maximum thickness Ht andthus a strength of the bottom is enhanced corresponding to thethickness. Due to these together with the lower thrust load, whendrilling is performed on the powder-compact green body G using the Rdrill 10, even if a thickens of a bottom of a through-hole when thedrill penetrates becomes thin, it is possible to facilitate the bottomto remain in place against the thrust load and thus to allow the drillto perform cutting until just before the drill penetrates therethrough.Therefore, it is possible to suppress the bottom from collapsing withoutbeing cut before the drill penetrates therethrough and thus to suppressoccurrence of edge chipping.

These have an effect even on a blind hole just before penetration, otherthan a trough-hole. Specifically, even in the case of a blind hole inwhich a thickness of a bottom of the hole (minimum thickness as measuredfrom a bottom surface of the hole to the opposite surface) is thin, theR dill causes the bottom surface of the hole to be formed in ahemispheric shape, thereby enhancing a strength of the bottom. Namely,even if a thickness of a bottom of a hole is thin, it is possible tosuppress the bottom from collapsing and thus to form a blind hole, ofwhich a bottom has a thinner thickness. For example, machining can beperformed until a thickness of the bottom becomes ½ of a drill diameter(hole diameter), further ¼ of the drill diameter.

On the other hand, when a drill having a V-shaped cutting edge(hereinafter, often referred to as a “V drill”), which is conventionallywidely used, is employed, it is difficult to suppress occurrence of edgechipping. Views on the right side FIG. 1 show a drill (V drill) 11, inwhich a shape of a cutting edge thereof is a V shape, and a workmaterial (powder-compact green body) G, on which drilling has beenperformed using the V drill 11. Like the R drill 10, the V drill 11illustrated in FIG. 1 is simply shown by omitting grooves and the like.Also, in this example, the V drill 11 is configured so that a pointangle β of a cutting edge 110 thereof is in the order of 130° to 140°and a drill diameter d is equal to that of the R drill 10.

As shown in two views on the right lower side of FIG. 1, when drillingis performed on a powder-compact green body G using the V drill 11, theshape of the cutting edge 110 is transferred to the powder-compact greenbody G, so that a hole, of which a bottom has a cross section of atriangle shape, i.e., a conical shape, is formed in the powder-compactgreen body G. In the V drill 11, since the shape of the cutting edge 110is the V shape (triangle shape), as indicated by solid arrows in thefigure, a thrust load is exerted in a direction perpendicular to sidesof the triangle (conical surface). Meanwhile, in the powder-compactgreen body G, as shown in solid arrows in the figure, the conical bottomsurface of the hole bears a thrust load from the drill by sides of thetriangle (conical surface), and thus stress is concentrated at apex atwhich sides intersect with each other. Namely, the conical bottomsurface of the hole causes a stress concentration and has a lowerstrength, as compared with a hemispheric bottom surface of the hole.

In other words, when drilling is performed on the powder-compact greenbody G using the V drill 11, the V drill 11 cannot disperse the thrushload exerted on the bottom cannot be dispersed, as compared with the Rdrill 10, and thus the bottom is likely to be chipped.

Also, in the case of the V drill 11, a maximum thickness of the bottomsurface of the hole is smaller and thus a strength of the bottom isdecreased corresponding to the thickness. Therefore, when drilling isperformed on the powder-compact green body G using the V drill 11, ifthe thickness of the bottom when the drill penetrates becomes thin, thebottom is likely to collapse without being cut before the drillpenetrates therethrough. Accordingly, it is difficult to suppressoccurrence of edge chipping.

Meanwhile, the V drill is generally used upon drilling. The reason isthat a sharp point of the V drill can be thrust at the center of a holeto be machined into a workpiece and thus the drill can be preciselypositioned at the center of the hole. Although attempting to use the Rdrill for drilling, the R drill cannot be thrust into a workpiece andthus cannot be precisely positioned. For this reason, the V drill isgenerally used upon drilling.

However, according to the present invention, drilling is performed on apowder-compact green body having a lower strength. Since the strength ofthe powder-compact green body is lower, the present inventors have foundthat even in the case of the R drill, a point thereof can be also thrustinto the powder-compact green body and thus be precisely positioned. Forthis reason, the present inventors have found that drilling can beperformed by the R dill and then that if drilling is performed using theR drill, it is possible to prevent burrs from occurring as describedabove and the like.

The present inventors have obtained the above findings and completed thepresent invention. First, exemplary embodiments of the present inventionwill be listed and described.

(1) A method for manufacturing a sintered component according to oneaspect of the present invention includes a molding step, a drilling stepand a sintering step. The molding step is configured to press-mold a rawmaterial powder containing a metal powder and thus to fabricate apowder-compact green body. The drilling step is configured to form ahole in the powder-compact green body using a drill. The sintering stepis configured to sinter the powder-compact green body after drilling.The drill used for drilling has a circular-arc shaped cutting edge on apoint portion thereof.

According to the method for manufacturing a sintered component asdescribed above, drilling using a drill is performed on a powder-compactgreen body before sintering. Accordingly, cutting can be easilyperformed and also a cutting resistance (thrust load) can besignificantly reduced. Therefore, as compared with a conventionalmanufacturing method in which drilling using a drill is performed aftersintering, a machining time can be reduced, a precision of a machinedhole can be enhanced and also a tool life can be significantly improved.Further, when drilling using a drill is performed on the powder-compactgreen body, burrs hardly occur. Even if burrs have occurred, the burrscan be easily removed, for example, by an air blower, and thus time andefforts required for a bur removing operation can be reduced. The “hole”includes a through-hole (open hole), which extends throughout, or ablind hole, which does not extend throughout.

Further, in the method for manufacturing a sintered component asdescribed above, drilling is performed using the drill, in which has thecircular arc-shaped cutting edge on the point portion thereof.Accordingly, when a through-hole is formed in the powder-compact greenbody, occurrence of edge chipping can be suppressed. As a result, themethod for manufacturing a sintered component as described above cansuppress occurrence of edge chipping and also has a good productivity.

Meanwhile, as used herein, the term “shape of the cutting edge” refersto a projection shape of the cutting edge as projected to a plane, whichpasses through a center axis of the drill and also is parallel to thecenter axis, in a direction perpendicular thereto in a state where thecutting edge are arranged to be parallel to the parallel plane

Also, the term “circular arc-shaped cutting edge” means that aprojection shape of a cutting edge is a circular arc shape (see a viewon the left upper side of FIG. 3). In the case where a shape of thecutting edge is a circular arc shape, when viewing the drill, which isbeing rotated, from a direction perpendicular to a rotation axis of thedrill, a rotational trajectory of the cutting edge is looked like acircular arc shape.

(2) In one mode of the method of manufacturing a sintered component asdescribed above, the drill is configured so that a rake angle of thecutting edge may be more than 0° and 10° or less.

In view of suppression of occurrence of edge chipping, it is thoughtthat a smaller cutting resistance (thrust load) is favorable. Since therake angle of the cutting edge is more than 0° and 10° or less, a thrustload can be reduced and thus occurrence of edge chipping can be moreeffectively suppressed. Since the rake angle is more than 0°, thecutting edge becomes sharp and thus a thrush load is reduced. On theother hand, if the rake angle is increased, an edge strength isdecreased as the cutting edge becomes sharp. However, since a workmaterial to be machined is a powder-compact green body, chipping due toa decrease in edge strength hardly occurs. In view of ensuring an edgestrength, the rake angle preferably is more than 0.01°, more preferably0.1°.

If the rake angle is more than 10°, the thrust load is increased.Therefore, the rake angle preferably is 10° or less. In view of reducingthe thrust load, the rake angle is more preferably, for example, 5° ormore and 8° or less.

As used herein, the term “rake angle” refers to an angle γ definedbetween a plane parallel to the center axis of the drill and a rake facein a state where the cutting edge are arranged to be parallel to theparallel plane, as shown in a view on the right lower side of FIG. 3.

(3) In one mode of the method of manufacturing a sintered component asdescribed above, the drill may be configured so that a circular arcdefining the cutting edge has a central angle of 135° or more and 180°or less.

Since the circular arc defining the cutting edge has a central angle of135° or more and 180° or less, occurrence of edge chipping can besufficiently suppressed. If the central angle of the circular arc-shapedcutting edge is 135° or more, the shape of the cutting edge becomesclose to a semi-circular shape, thereby causing a thrust load to bedispersed in a radial shape. Therefore, it is possible to enhance theeffect of reducing a thrust load and also to cause a thrust load duringdrilling to be dispersed. Further, a shape of a bottom surface of thehole becomes close to a hemispheric shape, thereby enhancing a strengthagainst a thrust load. In addition, a maximum thickness Ht of the bottom(see views on the left side of FIG. 1) is increased. Correspondingly, astrength of the bottom is enhanced and thus the bottom is hardlychipped. The central angle of the circular arc is more preferably, forexample, 150° or more, particularly preferably 180° to obtain asemi-circular cutting edge.

On the other hand, a radius of the circular arc defining of the cuttingedge preferably is generally equal to a radius of a drill diameter, forexample, preferably 0.4 times or more and 0.6 times or less of the drilldiameter. In particular, the shape of the cutting edge preferably is asemi-circular shape, and preferably also, the central angle of thecircular arc is 180° and the radius of the circular arc is 0.5 times ofthe drill diameter, i.e., equal to a radius of the drill diameter. Asused herein, the “diameter of drill (or drill diameter)” refers to anouter diameter dimension of a portion on which the cutting edge isformed (so-called edge portion).

(4) A sintered component according to one aspect of the presentinvention is a sintered component having a hole formed therein. An innercircumferential surface of the hole has a satin finish shape.

When drilling using a drill is performed on a powder-compact green bodybefore sintering as described above, bonding between metal powderparticles is weak and thus a hole is formed as metal powder particlesare cut while being scrapped by the drill. Accordingly, the innercircumferential surface of the hole formed in the powder-compact greenbody has a satin finish shape in which concave and convex portions dueto particles are formed overall. Since such a surface aspect of theinner circumferential surface of the hole is substantially maintainedeven after sintering, an inner circumferential surface of the hole ofthe sintered component, which is obtained by sintering thepowder-compact green body having the hole formed therein, has also asatin finish shape. In other words, the fact that the innercircumferential surface of the hole formed in the sintered component hasthe satin finish shape means that drilling using a drill is performed onthe powder-compact green body before sintering. Such a sinteredcomponent, in which an inner circumferential surface of a hole thereofhas a satin finish shape, has a good productivity, as compared withconventional sintered components, in which a hole is formed aftersintering.

In contrast, when drilling using a drill is performed after sintering,bonding between metal powder particles is strong due to sintering andthus a hole is formed as a lump of metal a metal is cut by the drill.Accordingly, the inner circumferential surface of the hole formed bydrilling the sintered component using a drill has an overall smoothsurface having a few of concave and convex portions and thus becomes ashiny state.

(5) In one mode of the sintered component as described above, a tenpoint medial height Rz of the inner circumferential surface of the holemay be 20 μm or more.

When a hole is formed in a powder-compact green body by a drill beforesintering and then the powder-compact green body is sintered, a tenpoint median height Rz of an inner circumferential surface of a holeformed in the resulting sintered component may be for example 20 μm ormore, although varying depending on shapes/sizes of metal powderparticles. An upper limit of the ten point median height Rz of the innercircumferential surface of the hole may be, for example, 150 μm or less.In contrast, if a hole is formed by a drill after sintering, a ten pointmedian height Rz of an inner circumferential surface of the hole formedin the sintered component is typically smaller than 20 μm, further 15 μmor less.

(6) A drill according to one aspect of the present invention is a drillfor drilling a work material. The work material is a powder-compactgreen body fabricated by press-molding a raw material powder containinga metal powder. The drill has a circular arc-shaped cutting edge on apoint portion thereof.

According to the drill as described above, it is possible to suppressoccurrence of edge chipping when a through-hole is formed in apowder-compact green body, since the circular arc-shaped cutting edge isprovided on the point portion.

[Details of Exemplary Embodiments of the Invention]

Now, specific examples of a method for manufacturing a sinteredcomponent, a sintered component and a drill according to exemplaryembodiments of the present invention will be described with reference tothe accompanying drawings. Meanwhile, it should be noted that thepresent invention is intended not to be limited to such examples, but tobe defined by the appended claims and also to encompass all of changeswithin the meaning and scope of equivalency of the claims.

<Method for Manufacturing a Sintered Component>

A method for manufacturing a sintered component according to anembodiment of the present invention includes a molding step offabricating a powder-compact green body; a drilling step of forming ahole in the powder-compact green body using a drill; and a sinteringstep of sintering the powder-compact green body after drilling. One offeatures of the method for manufacturing a sintered component is that adrill having a circular arc-shaped cutting edge on a point portionthereof is used in the drilling step. Hereinafter, each step of themanufacturing method will be described in detail, principally referringto FIG. 2.

(Molding Step)

In the molding step, a raw material powder containing a metal powder ispress-molded to fabricate a powder-compact green body G (see a view onthe top of FIG. 2). The powder-compact green body G is a material for asintered component and thus is formed to have a shape corresponding to asintered component S (see a view on the bottom of FIG. 2) to bemanufactured. Herein, as the powder-compact green body G (sinteredcomponent S), a cylindrical body in which an circular axial bore 30 isformed at the center thereof will be described by way of example.

<Raw Material Powder>

The raw material powder essentially contains a metal powder. A materialfor the metal powder can be properly selected depending on a material ofa sintered component to be manufactured and may typically includeiron-based materials. The “iron-based materials” mean iron or ironalloy, whose main constituent is iron. The iron alloy includes alloycontaining one or more additive elements selected, for example, from Ni,Cu, Cr, Mo, Mn, C, Si, Al, P, B, N and Co. Specifically, the iron alloyincludes stainless steel, Fe—C alloy, Fe—Cu—Ni—Mo alloy, Fe—Ni—Mo—Mnalloy, Fe—P alloy, Fe—Cu alloy, Fe—Cu—C alloy, Fe—Cu—Mo alloy,Fe—Ni—Mo—Cu—C alloy, Fe—Ni—Cu alloy, Fe—Ni—Mo—C alloy, Fe—Ni—Cr alloy,Fe—Ni—Mo—Cr alloy, Fe—Cr alloy, Fe—Mo—Cr alloy, Fe—Cr—C alloy, Fe—Ni—Calloy, Fe—Mo—Mn—Cr—C alloy and the like. By essentially containing aniron-based material powder, an iron-based sintered component isobtained. If an iron-based material powder is essentially contained, acontent thereof may be set to, for example, 90 mass % or more, further95 mass % or more, assuming that the raw material powder is 100 mass %.

When an iron-based material powder, in particular an iron powder isessentially contained, metal powders, such as Cu, Ni and Mo, may beadded as alloy constituents. Cu, Ni and Mo are elements intended toenhance hardenability, and an amount of addition thereof may be set to,for example, more than 0 mass % and 5 mass % or less, further 0.1 mass %or more and 2 mass % or less, assuming that the raw material powder is100 mass %. Also, a nonmetallic inorganic material, such as carbon(graphite) powder may be added. C is an element intended to enhancestrength of a sintered body or heat-treated body, and a content thereofmay be set to, for example, more than 0 mass % and 2 mass % or less,further 0.1 mass % or more and 1 mass % or less, assuming that the rawmaterial powder is 100 mass %.

Preferably, the raw material powder contains a lubricant. By containingthe lubricant in the raw material powder, when the raw material powderis press-molded to fabricate a powder-compact green body, lubricity uponmolding can be increased and thus moldability can be enhanced.Therefore, even if a pressure for press-molding is lower, a densifiedpowder-compact green body G can be easily obtained and thus ahigh-density sintered component S can also be easily obtained. Further,if the lubricant is mixed with the raw material powder, the lubricant isdispersed inside the powder-compact green body G and thus also serves asa lubricant for a drill when the powder-compact green body G is drilledusing the drill 10 in the subsequent step (see a view on the middle ofFIG. 2). Therefore, a cutting resistance (thrust load) can be reduced ora tool life can be improved. For example, the lubricant includes metalsoaps, such as zinc stearate and lithium stearate; fatty acid amidessuch as stearic acid amide; higher fatty acid amides such asethylene-bis-stearic acid amide and the like. The lubricant may take anyform, such as solid form, powder form or liquid form. A content of thelubricant may be set to, for example, 2 mass % or less, further 1 mass %or less, assuming that the raw material powder is 100 mass %. If acontent of the lubricant is 2 mass % or less, it is possible to increasea proportion of metal powder to be contained in a powder-compact greenbody G. Accordingly, even if a pressure for press-molding is lower, adensified powder-compact green body G can be easily obtained. Further,it is possible to suppress a volumetric shrinkage due to dissipation ofthe lubricant when the powder-compact green body G is sintered in thesubsequent step. As a result, a high-density sintered component S havingan enhanced dimension precision can be easily obtained. From the pointof view that the effect of enhancing lubricity is obtained, the contentof the lubricant is preferably set to 0.1 mass % or more, further 0.5mass % or more.

For this reason, the content of the lubricant may be preferably set to0.1 mass % or more and 2 mass % or less, further 0.5 mass % or more and2 mass % or less, 0.1 mass % or more and 1 mass % or less, even 0.5 mass% or less and 1 mass % or less, assuming that the raw material powder is100 mass %.

The raw material powder contains no organic binder. Since no organicbinder is contained in the raw material powder, a proportion of metalpowder to be contained in a powder-compact green body can be increased.Accordingly, even if a pressure for press-molding is lower, a densifiedpowder-compact green body G can be easily obtained. In addition, thereis no need to degrease the powder-compact green body G in the subsequentstep.

The raw material powder essentially consists of the metal powder asdescribed above and is also permitted to contain inevitable impurities.

As the metal powder described above, water atomized powder, reductionpowder, gas atomized powder and the like may be employed, and amongothers, water atomized powder or reduction powder are preferable. Thewater atomized powder or reduction powder has a lot of concave andconvex portions formed on a surface of particles. Accordingly, concaveand convex portions of particles are engaged with each other duringmolding, thereby enhancing a shape retaining ability of thepowder-contact green body G. In general, from the gas atomized powder,particles having a few of concave and convex portions on a surfacethereof are apt to be obtained, whereas from the water atomized powderor reduction powder, particles having a lot of concave and convexportions on a surface thereof are apt to be obtained. Also, an averageparticle diameter of the metal powder may be, for example, 20 μm ormore, 50 μm or more and 150 μm or less. The “average particle diameterof the metal powder” is a particle diameter (D50), at which a cumulativevolume in a volumetric particle size distribution as measured by a laserdiffraction particle size measuring device becomes 50%. So long as theaverage particle diameter of the metal powder is within the above range,treating thereof is easy and thus press-molding is facilitated.

<Press-Molding>

For press-molding, a molding apparatus (mold), by which a shapecorresponding to a shape of a sintered component as a final product canbe molded, is employed. In the cylindrical powder-compact green body Gas illustrated in FIG. 2, an axial bore 30 is integrally formed duringmolding. The powder-compact green body G may be formed, for example,using upper and lower punches having a circular ring-shaped pressingsurface for forming both end surfaces of the powder-compact green bodyG, a circular columnar-shaped inner die configured to be inserted intothe insides of the upper and lower punches for forming an innercircumferential surface of the powder-compact green body G, and an outerdie configured to surround outer circumference surfaces of the upper andlower punches and having an circular insertion hole formed therein forforming an outer circumferential surface of the powder-compact greenbody G. Both axial end surfaces of the powder-compact green body G aresurfaces, which are pressed by the upper and lower punches, the innerand outer circumferential surfaces thereof are surfaces in slidingcontact with the inner and outer dies, and the axial bore 30 isintegrally formed during molding. A pressure for press-molding may be250 MPa or more and 800 MPa or less

[Drilling Step]

In the drilling step, a hole 50 is formed in the powder-compact greenbody G using a drill 10 (see a view on the middle of FIG. 2). The hole50 may be a through-hole or blind hole. Herein, a through-hole, whichextends from the outer circumferential surface to the innercircumferential surface of the powder-compact green body G, is formedusing the drill 10. Namely, the axial bore (molded hole) 30, which ismolded in the powder-compact green body G, and the through-hole (drilledhole) 50, which is formed using the drill 10, are connected with eachother, and thus an opening on an exit side of the through-hole 50 isprovided on the inner circumferential surface (inner circumferentialsurface of the axial bore 30) of the powder-compact green body G. Inthis example, the through-hole 50 is formed at a location where adistance (thickness) between an inner circumferential surface of thethrough-hole 50 and an outer surface (end surface) of the powder-compactgreen body G is equal to or more than a diameter of the through-hole 50.The drill 10 used for drilling of the powder-compact green body G willbe described with reference to FIG. 3.

<Drill>

A view on the left upper side of FIG. 3 is a schematic plan view of thedrill, a view on the left lower side of FIG. 3 is a schematic front viewof the drill as viewed from a point side thereof, and a view on theright lower side of FIG. 3 is a schematic side view partially showing apoint portion of the drill. The drill 10 is intended to drill into awork material. The work material is the powder-compact green body G (seethe view on the middle of FIG. 2) obtained by press-molding a rawmaterial powder containing a metal powder. The drill 10 is configuredaccording to an embodiment of the present invention.

The drill 10 as illustrated in FIG. 3 is a so-called R drill, which hasa circular arc-shaped cutting edge 110 on a point portion 100 thereof.The point portion 100 is a portion ranging from a point (apex) of thecutting edge 110 to outer corners 120.

<Shape of Cutting Edge>

As shown in the view on the left upper side of FIG. 3, the drill 10 isconfigured so that the cutting edge 110 has a circular arc-shapedprojection shape, as viewed in a plan view from a directionperpendicular to a plane parallel to a center axis of the drill 10 in astate where the cutting edge 110 are in turn arranged to be parallel tothe parallel plane.

A central angle α of a circular arc defining the cutting edge 110 is,for example, 130° or more, preferably, 135° or more and 180° or less,more preferably 150° or more. In this example, the central angle α ofthe circular arc is 180°.

A radius R of the circular arc defining the cutting edge is, forexample, 0.4 times or more and 0.6 times or less of a drill diameter dand preferably equal to 0.5 times of a diameter d of the drill, i.e., aradius (d/2) of the drill diameter d. In this example, a shape of thecutting edge is a circular arc shape, a central angle of the circulararc is 180°, and a radius R of the circular arc is equal to the radiusof the drill diameter d. For example, the diameter d of the drill 10 is,but not particularly limited to, 1.0 mm or more and 20.0 mm or less.

<Rake Angle of Cutting Edge>

A rake angle of the cutting edge 110 is, for example, 0° or more,preferably more than 0° and 10° or less, more preferably 5° or more and8° or less. As shown in the view on the right lower side of FIG. 3, therake angle of the cutting edge 110 is an angle γ defined between a planeP parallel to the center axis of the drill 10 and a rake face 111defining the cutting edge 110, as viewed as viewed in a side view from adirection perpendicular to the center axis of the drill 10 and alsoparallel to a horizontal plane in a state where the cutting edge 110 arein turn arranged to be parallel to the plane parallel to the axis. Inthis example, the rake angle of the cutting edge 110 is 7°.

<Cutting Condition>

Cutting conditions, such as the number of revolutions or a feed rate (orfeed amount) of the drill 10, may be properly set depending on amaterial of the powder-compact green body G (metal powder), a depth ofthe through-hole 50 to be formed, or a diameter of the drill 10 (seeFIG. 2). For example, the number of revolution may be set to 1000 rpm ormore, further 2000 rpm or more, the feed rate may be set to 100 mm/minor more, further 200 mm/min or more, and the feed amount may be set to0.01 mm/rev. or more, further 0.1 mm/rev. or more. It is proved throughexperiments that if a powder-compact green body is machined, machiningcan be performed at a higher speed, as compared with the case where asintered body is machined.

The inner circumferential surface of the hole (through-hole) 50 formedin the powder-compact green body G using the drill 10 has a satin finishshape. Bonding between metal powder particles in the powder-compactgreen body G is weak. Therefore, when drilling is performed using thedill, the through-hole 50 is formed as metal powder particles are cutwhile being scrapped by the drill. Accordingly, the innercircumferential surface of the through-hole 50 formed in thepowder-compact green body G has such a satin finish shape in whichconcave and convex portions due to particles are formed overall.

(Sintering Step)

In the sintering step, the powder-compact green body G after drilling issintered. For sintering, a sintering furnace (not shown) capable ofcontrolling a temperature atmosphere is used. Sintering conditions maybe properly selected from any conditions required for sinteringdepending on a material of the powder-compact green body G (metalpowder) and the like. A sintering temperature may be set to, forexample, 1000° C. or more, further 1100° C. or more, 1200° C. or more,and also set to a temperature (e.g., 1400° C. or less) equal to or lowerthan a melting point of a main metal powder. A sintering time may be setto, for example 15 minutes or more and 150 minutes or less, further 20minutes or more and 60 minutes or less. By sintering, a sinteredcomponent S having a hole (through-hole) 505 is obtained (see a view onthe bottom of FIG. 2). The sintered component S is configured accordingto an embodiment of the present invention.

<Sintered Component>

In the sintered component S, the hole (through-hole) 505 is formed. Thethrough-hole 505 is the through-hole 50 formed in the powder-compactgreen body G by drilling using the drill 10 before sintering (see a viewon the middle of FIG. 2). As described above, the inner circumferentialsurface of the through-hole 50 formed in the powder-compact green body Gusing the drill 10 has the satin finish shape. Since such a surfaceaspect of the inner circumferential surface of the through-hole 50 issubstantially maintained even after sintering, an inner circumferentialsurface of the through-hole 505 of the sintered component S, which isobtained by sintering the powder-compact green body G, has also a satinfinish shape. In other words, the fact that the inner circumferentialsurface of the through-hole 505 formed in the sintered component S hasthe satin finish shape means that drilling using the drill 10 isperformed on the powder-compact green body G before sintering. In thesintered component S, a ten point medial height Rz of the innercircumferential surface of the through-hole 50 may be, for example, 20μm or more or 150 μm or less.

In this example, the through-hole 50S is formed at a location where adistance (thickness) between the inner circumferential surface of thethrough-hole 50S and an outer surface (end surface) of the sinteredcomponent S is equal to or more than a diameter of the through-hole 50S.

[Action and Effects]

In the method for manufacturing a sintered component according to theforegoing embodiments, drilling using a drill is performed on apowder-compact green body before sintering. Accordingly, cutting can beeasily performed and also a cutting resistance (thrust load) can besignificantly reduced. Therefore, as compared with a conventionalmanufacturing method in which drilling using a drill is performed aftersintering, a machining time can be reduced, a precision of a machinedhole can be enhanced and also a tool life can be significantly improved.Further, in the method for manufacturing a sintered component accordingto the above embodiments, drilling is performed using a drill, which hasa circular arc-shaped cutting edge on a point portion thereof.Accordingly, when a through-hole is formed in the powder-compact greenbody, occurrence of edge chipping can be suppressed. As a result, themethod for manufacturing a sintered component as described above cansuppress occurrence of edge chipping and also has a good productivity.

In the sintered component according to the foregoing embodiments, thehole (through-hole) is formed therein and the inner circumferentialsurface of the hole has the satin finish shape. Accordingly, since thismeans that drilling using a drill is performed on a powder-compact greenbody before sintering, the sintered component has a good productivity.

In the drill according to the foregoing embodiments, the circulararc-shaped cutting edge is provided on the point portion thereof.Accordingly, it is possible to suppress occurrence of edge chipping whena through-hole is formed in a powder-compact green body.

Although in the foregoing embodiments, the case where the though-hole isformed in the powder-compact green body using the drill has beendescribed by way of example, a hole to be formed may be a blind hole. Inthe case of the blind hole, it is possible to make a thickness of abottom of the hole thinner. For example, the drill is suitable for acase where a blind hole is formed such that a thickness of a bottomthereof is 2 times or less of a drill diameter (hole diameter). A lowerlimit of the thickness of the bottom may be set to about ¼ or more ofthe drill diameter (hole diameter), or about ½ or more.

Test Example 1

Powder-compact green bodies were fabricated by press-molding a rawmaterial powder containing a metal powder and then a drilling test wasperformed on the powder-compact green bodies using drills havingdifferent cutting edge shapes.

(Powder-Compact Green Body)

A water atomized iron powder (average particle diameter (D50): 100 μm),a water atomized copper powder (average particle diameter (D50): 30 μm),a carbon (graphite) powder (average particle diameter (D50): 20 μm) andethylene-bis-stearic acid amide as a lubricant were prepared, and thenby mixing them, a raw material powder was prepared.

The prepared raw material powder was filled in a predetermined mold andthen press-molded at a pressing pressure of 600 MPa. In this way,plate-shaped powder-compact green bodies having a size of length 50mm×width 20 mm×thickness 10 mm were fabricated. A density ofpowder-compact green bodies was 6.9 g/cm³. This density is an apparentdensity as calculated from size and mass of the powder-compact greenbodies.

Subsequently, drilling was performed on the fabricated powder-compactgreen bodies using the drills, thereby forming a through-hole in athickness direction of the powder-compact green bodies. Then, an openingon an exit side of the through-hole was observed and an occurrencesituation of edge chipping was checked.

As one of drills, an R drill, in which a shape of a cutting edge thereofis semi-circular as shown in FIG. 3, was prepared. In the prepared Rdrill, a drill diameter d was 8.0 mm, a central angle α of a circulararc defining the cutting edge was 180°, and a radius R of the circulararc was 4.0 mm (0.5 times of the drill diameter d). Also, a rake angleof the cutting edge was 0°. The R drill was fabricated by grinding acutting edge on a point portion of a drill (Model No. MDW0800GS4,material: sintered carbide) produced by Sumitomo Electric Hardmetal Co.

Also, a V drill, in which a shape of a cutting edge thereof is aV-shape, was prepared. The prepared V drill was a drill (Model No.05WHNSB0400-TH, material: sintered carbide) produced by Hitachi ToolEngineering, Ltd. In the V drill, a drill diameter d was 4.0 mm and apoint angle of the cutting edge was 140°.

Drilling was performed on the powder-compact green bodies using the Rdrill and the V drill, thereby forming a through-hole. In the case ofusing the R drill, cutting conditions were the number of revolutions of4000 rpm and a feed rate of 1600 mm/min. In the case of using the Vdrill, cutting conditions were the number of revolutions of 4000 rpm, afeed rate of 800 mm/min from an entrance of the hole up to a hole depthof 5 mm, and then a feed rate of 1600 mm/min from the hole depth of 5 mmup to penetration.

After drilling, with respect to the powder-compact green bodies in whicha through-hole was formed using each of the drills, an opening on anentrance side of the through-hole was observed using an opticalmicroscope. The results are shown in FIGS. 4 and 5. FIG. 4 is the caseof using the R drill and FIG. 5 is the case of using the V drill.

In FIG. 4, a black annular portion, which has a constant width andborders a periphery of the central circular portion (through-hole), isan inner circumferential surface of the through-hole. As shown in FIG.4, it can be seen that if a through-hole is formed using the R drill, avery few of edge chippings occur on an opening on an entrance side ofthe through-hole, and in this example, no edge chipping can be founded.

In FIG. 5, a grey portion, which is spread around the through-hole, isan edge chipping. As shown in FIG. 5, it can be seen that if athrough-hole is formed using the V drill, a large edge chipping occurson an opening on an entrance side of the through-hole. Also, an amountof edge chipping in the case where the through-hole was formed by the Vdrill was measured as 1.55 mm. The amount of edge chipping was obtainedby measuring a distance from the center of the through-hole to theremotest point among points, which are located on a contour of an edgechipped portion, from the microscope image of FIG. 5, and thencalculating a difference between the distance and the diameter of thethrough-hole.

From this result, it can be seen that occurrence of edge chipping can besuppressed by using an R drill having a circular-arc shaped cuttingedge.

Test Example 2

Drilling was performed on powder-compact green bodies using R drillshaving different rake angles and then thrust loads when forming athrough-hole were compared with each other.

As powder-compact green bodies to be machined, the same as those in TestExample were employed.

Like Test Example 1, the R drills used had a cutting edge of asemi-circular shape and were fabricated by grinding a cutting edge on apoint portion of a drill (Model No. MDW0800GS4, material: sinteredcarbide) produced by Sumitomo Electric Hardmetal Co. In these R drills,a drill diameter d was 8.0 mm, a central angle α of a circular arcdefining the cutting edge was 180°, and a radios R of the circular arcwas 4.0 mm (0.5 times of the drill diameter d). In addition, three typesof R drills having rake angles of 0°, 7° and 10°, respectively, werefabricated. An R drill having a rake angle of 0° was referred to as R0,an R drill having a rake angle of 7° was referred to as R7, and an Rdrill having a rake angle of 10° was referred to as R10.

Drilling was performed three times on the powder-compact green bodiesusing each of the three types of drill (R0, R7, R10), thereby formingthree through-holes in a thickness direction of the powder-compact greenbodies. Cutting conditions were the number of revolutions of 2000 rpmand a feed rate of 200 mm/min (feed amount of 0.1 mm/rev). Also, foreach of the first to third drilling, thrust loads and torques whenforming a through-hole were measured. Thrust loads and torques weremeasured from beginning of drilling until a through-hole is formed,using a cutting dynamometer (Model No. 9272 produced by Kistler JapanCo., Ltd.), and maximum values thereof were obtained. Also, averagevalues were calculated from each thrust load and torque in each drillingprocess.

Thrust loads and torques when drilling was performed using the drillsR0, R7 and R10 are shown in Tables 1 to 3, respectively. For example,the reference character “R0-1” in Table 1 means the first drilling usingthe drill R0, and thus the symbol on the front half thereof indicatesthe drill used and the number on the rear half indicates the number oftimes of machining (the same is also applied to Tables 2 and 3).

Further, in FIG. 6, a relationship of a rake angle to a thrust load anda torque is shown based on the average values of thrust loads andtorques in each drill. In the graph of FIG. 6, a horizontal axisrepresents a rake angle (degree, °), a vertical axis on the left siderepresents a thrust load (N), a vertical axis on the right represents atorque (N·m), the symbol ▪ is a thrust load and the symbol ⋄ is atorque.

TABLE 1 Thrust Load Torque (N) (N · m) R0-1 49.15 0.33 R0-2 50.46 0.39R0-3 50.54 0.32 Ave. 50.05 0.35

TABLE 2 Thrust Load Torque (N) (N · m) R7-1 38.70 0.31 R7-2 40.20 0.34R7-3 41.59 0.30 Ave. 40.16 0.32

TABLE 3 Thrust Load Torque (N) (N · m) R10-1 47.89 0.27 R10-2 46.68 0.26R10-3 46.51 0.29 Ave. 47.03 0.27

From the results of Tables 1 to 3 and FIG. 6, it can be seen that athrust load on an R drill having a rake angle 7° is smaller than thoseon R drills having rake angles of 0° and 10°, respectively. It isthought that if a rake angle is within a range of more than 0° and 10°or less, a thrust load can be reduced as compared with the case where arake angle is 0°. Therefore, it is thought that occurrence of edgechipping can be more effectively suppressed by using an R drill having arake angle of more than 0° and 10° or less. On the other hand, it can beseen that a torque tends to be decreased as a rake angle is increased.

Test Example 3

After drilling is performed on a powder-compact green body using an Rdill, the powder-compact green body in which a through-hole is formed bythe R drill was sintered, thereby fabricating a sintered component.

As the powder-compact green body to be machined, the same as those inTest Example was employed.

Herein, an R drill in which a shape of a cutting edge thereof issemi-circular and a drill diameter d is 3.5 mm was employed. The R drillwas fabricated by grinding a cutting edge on a point portion of a drill(Model No. MDW0350GS4, material: sintered carbide) produced by SumitomoElectric Hardmetal Co. In the R drill, a central angle α of a circulararc defining the cutting edge was 180°, a radios R of the circular arcwas 1.75 mm (0.5 times of the drill diameter d), and a rake angle was0°.

Drilling was performed on the powder-compact green body using the Rdrill, thereby forming a through-hole in a thickness of thepowder-compact green body. Cutting conditions were the number ofrevolutions of 2000 rpm and a feed rate of 200 mm/min (feed amount of0.1 mm/rev). After drilling, the powder-compact green body, in which thethrough-hole was formed, was sintered at a temperature of 1130° C.during 20 minutes, thereby fabricating a sintered component.

A powder-compact green body in which a through-hole was formed in thesame manner, was cut along a thickness direction thereof through acenter axis of the through-hole, and an inner circumferential surface ofthe through-hole was observed by an optical microscope. A photograph ofthe cross section is shown in FIG. 7. A band-shaped portion laterallyextending in FIG. 7 is the inner circumferential surface of thethrough-hole. As shown in FIG. 7, a shape of the inner circumferentialsurface of the though-hole is a satin finish shape. Further, a ten pointmedian height Rz of the inner circumferential surface was measured as 40μm. Also, the sintered component fabricated as described above was cutalong a thickness direction thereof through a center axis of thethrough-hole, and an inner circumferential surface of the through-holewas observed by the optical microscope. The inner circumferentialsurface had the same surface aspect as that of the inner circumferentialsurface of the through-hole of the powder-compact green body asdescribed above, and also a ten point median height Rz thereof was thesame. The ten point median heights Rz were values as measured inaccordance with the standard “Geometrical Product Specifications(GPS)—Surface texture: Profile method—Terms, definitions and surfacetexture parameters JIS B 0601:2013”.

A through-hole was formed in a sintered component after sintering, usinga drilling and then in the same manner, an inner circumferential surfaceof the through-hole was observed. As a result, although not shown, theinner circumferential surface of the through-hole was a smooth surfacehaving a few of concave and convex portions and thus was a shiny state.Also, a ten point medial height Rz of the inner circumferential surfaceof the through-hole was measured as 11 μm. The drill used for drillingof the sintered component was MDW0350GS4 produced by Sumitomo ElectricHardmetal Co., in which a shape of a cutting edge thereof was a V shape,a drill diameter d was 3.5 mm and a point angle of the cutting edge was135°.

This application is based on Japanese Patent Application No. 2014-252531filed on Dec. 12, 2014, the entire contents of which are incorporatedherein by reference.

INDUSTRIAL APPLICABILITY

The method for manufacturing a sintered component according to oneaspect of the present invention can be used for manufacturing varioussintered components, such as automobile parts and machine parts(sprockets, rotors, gears, rings, flanges, pulleys, vanes, bearings andlike). The sintered component according to one aspect of the presentinvention can be used for various sintered components, such asautomobile parts and machine parts. The drill according to one aspect ofthe present invention can be used for drilling of a powder-compact greenbody.

REFERENCE NUMERALS LIST

-   -   10 Drill (R drill)    -   11 Drill (V drill)    -   100 Point portion    -   110 Cutting edge    -   111 Rake face    -   120 Outer corner    -   30 Axial bore    -   50 Hole (through-hole)    -   50S Hole (through-hole)    -   G Powder-compact green body (work material)    -   S Sintered component

The invention claimed is:
 1. A drill for drilling a powder-compact greenbody, comprising: a circular arc-shaped cutting edge only on a pointportion thereof, wherein the powder-compact green body is fabricated bypress-molding a raw material powder containing a metal powder and apowder lubricant, wherein the powder-compact green body is formed beforea heat treatment, wherein the lubricant is served as a lubricant for thedrill when the powder-compact green body is drilled to reduce a cuttingresistance, wherein a radius of the circular arc defining the cuttingedge is a radius of the drill, wherein a length of the point portionalong an axial direction of the drill is equal to the radius of thecircular arc, wherein the cutting edge cuts the powder-compact greenbody to form a hole in the powder-compact green body, and wherein ashape of an inner circumferential surface of the hole is a satin finishshape.
 2. The drill according to claim 1, wherein the drill isconfigured so that a rake angle of the cutting edge is more than 0° and10° or less.
 3. The drill according to claim 2, wherein the drill isconfigured so that a circular arc defining the cutting edge has acentral angle of 135° or more and 180° or less.
 4. The drill accordingto claim 2, wherein the rake angle is an angle between a center axis ofthe drill and the cutting edge.
 5. The drill according to claim 1,wherein a feed rate of the drill is 200 mm/min or more.
 6. The drillaccording to claim 1, wherein content of the lubricant is set to 0.1mass % or more and 2 mass % or less assuming that the raw materialpowder is 100 mass %.
 7. The drill according to claim 1, wherein thepoint portion is from an apex of the cutting edge to an outer corner. 8.The drill according to claim 1, wherein a feed amount of the drill is0.1 mm/rev or more.